Journal of Asian Earth Sciences 96 (2014) 84–92

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Journal of Asian Earth Sciences

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Clay mineralogy of the riverine sediments of Hainan Island, South ChinaSea: Implications for weathering and provenance

http://dx.doi.org/10.1016/j.jseaes.2014.08.0361367-9120/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors. Address: Key Laboratory of Marine HydrocarbonResources and Environmental Geology, Ministry of Land and Resources, QingdaoInstitute of Marine Geology, Fuzhou South Road 62#, Qingdao, Shandong 266071,China. Tel.: +86 53285718613 (B. Hu), +86 532 85776342 (J. Li); fax: +86 53285720553.

E-mail addresses: bangqihu@gmail.com (B. Hu), junli741001@gmail.com (J. Li).

Bangqi Hu a,⇑, Jun Li a,⇑, Ruyong Cui a, Helong Wei a, Jingtao Zhao a, Guogang Li b, Xisheng Fang c,Xue Ding a, Liang Zou a, Fenglong Bai a

a Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources, Qingdao Institute of Marine Geology, Qingdao 266071, Chinab Marine Engineering and Prospecting Institute of North China Sea, North China Sea Branch of the State Oceanic Administration, Qingdao 266033, Chinac Key Laboratory of Marine Sedimentology and Environmental Geology, First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China

a r t i c l e i n f o

Article history:Received 26 February 2014Received in revised form 26 August 2014Accepted 27 August 2014Available online 6 September 2014

Keywords:Clay mineralsProvenanceWeatheringHainan IslandSouth China Sea

a b s t r a c t

Clay mineralogy of 54 fluvial samples collected from 20 major rivers on Hainan Island are investigated inorder to determine compositional changes of clay minerals and to assess the weathering processes. Theclay mineral assemblages consist dominantly of kaolinite (31–66%), with a lesser abundance of chlorite(22–44%) and illite (4–33%), and a trace amount of smectite (0–15%). Fluvial sediments from the eastand northwest of Hainan Island are characterized by a higher kaolinite content and illite chemical indexand poorer illite crystallinity than those from southwest Hainan. Only minor smectite (mean of 7%)occurs in the sediments from west Hainan; smectite is total lacking in east Hainan. Compared with theadjacent basins, Hainan Island is characterized by moderate to intensive chemical weathering withstrong hydrolysis. Our results suggest that rainfall is the principal factor controlling the intensity ofchemical weathering on Hainan Island, with more intense chemical weathering occurring in easternand northwestern Hainan. Another practical implication of this study is that it provides a ‘‘missing’’end member (Hainan Island) in the provenance discrimination study focused on the northern SouthChina Sea (SCS). Hainan fine-grained sediments likely play an important role in providing clay mineralsto the northern SCS carried by the South China Sea Warm Current (SCSWC) during the summer.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The South China Sea (SCS) is the largest marginal sea in thewestern Pacific. It is bordered by the Asian continent and TaiwanIsland to the north and west and by the Philippine Islands and Borneoto the east and south (Fig. 1a). Numerous rivers, including bothlarge rivers (i.e., Pearl, Red, and Mekong Rivers) and small moun-tainous rivers in southwestern Taiwan, the Malay Peninsula, andBorneo, annually deliver huge amounts of fine-grained sedimentsto the SCS (Milliman and Farnsworth, 2011), making it as a signif-icant sediment sink. Specifically, the predominant sedimentsources in the northern SCS are the Pearl River, SW Taiwan andthe Luzon Islands, which together deliver more than 265 Mt/yr ofsediments to the sea (Liu et al., 2008, 2010b). These river-derived

terrigenous sediments have formed high sedimentation-ratedeposits in the geological past, especially sediment drifts on thenorthern slope of SCS (Bühring et al., 2004). Therefore, it is theideal area to study high-resolution paleoenvironmental changesincluding the East Asian monsoon evolution (Boulay et al., 2003,2005; Liu et al., 2003, 2004; Tamburini et al., 2003; Wan et al.,2008, 2010b), the uplift of the Himalaya (Wan et al., 2007, 2012),and the continental weathering history (Hu et al., 2012; Wanet al., 2009, 2012). Addressing the above information first requiresin-depth understanding of the sediment provenance and its possi-ble temporal variability. Accordingly, the sediment sources andtransport pathways in the northern SCS, both in the present andin geological history, have thus attracted broad attention (Liuet al., 2003, 2005, 2008, 2010b, 2010c; Liu et al., 2010a, 2011,2012a, 2013; Li et al., 2012b; Wan et al., 2010a).

Clay minerals are widely distributed in various sediment types,and their compositions have been used extensively to constrain theprovenance of fine-grained terrigenous sediment, to decipherthe climatic changes in the source area, and to illustrate thechanges of transport agents (ocean currents, winds, ice drifts)(Chamley, 1989; Fagel, 2007; Singer, 1980; Thiry, 2000; Velde,

Fig. 1. (a) Sketch map of the South China Sea and surrounding basins. (b) Locations of the Hainan riverine samples and averaged clay mineral assemblages of three riversystems. See Table 1 for detailed clay mineral contents of each river. S = smectite, I = illite, K = kaolinite, C = chlorite.

B. Hu et al. / Journal of Asian Earth Sciences 96 (2014) 84–92 85

1995). Recently, Liu and his colleagues conducted comprehensiveworks on the clay mineralogy and geochemistry of riverine sedi-ments surrounding the SCS, including the Pearl, Red, and MekongRivers in South China and the Indochina Peninsula (Liu et al.,2007a, 2007b), small mountainous rivers in southwestern Taiwan(Liu et al., 2008), major and moderate rivers in Luzon (Liu et al.,2009b), and small rivers in the Malay Peninsula, Borneo, and Suma-tra (Liu et al., 2012b; Wang et al., 2011), and the controlling mech-anisms of climatic, tectonic, and lithological forcing on the

weathering processes have been summarized. Based on the distinctclay mineral compositions of the surrounding basins in the SCS, thesource and transport of fine-grained sediments have been semi-quantitatively evaluated (e.g., Liu et al., 2008; Liu et al., 2011,2013).

However, few studies have been conducted on the weatheringproducts of Hainan Island, the second largest and highest islandin the northern SCS. The island covers an area of 33,000 km2 andhas likely served as an important sediment source for the

86 B. Hu et al. / Journal of Asian Earth Sciences 96 (2014) 84–92

surrounding rift basins (Lin et al., 2001; Xie et al., 2008; Yan et al.,2011). In this study, clay mineralogy has been investigated for thefirst time on the fluvial samples collected from 20 major rivers onHainan Island. Our objectives are threefold: (1) to reveal the claymineral assemblage characteristics; (2) to quantify chemicalweathering states; and (3) to discuss the implications for prove-nance discrimination in the northern SCS.

2. Geological and climatic settings

Hainan Island, the second largest island in China, is a continen-tal-type island in the north of the SCS, separated from MainlandChina by the Qiongzhou Strait (Fig. 1). Tectonically, it is locatedat the intersection of the Pacific oceanic plate, the Indochina Blockand the southern margin of the South China Block (Metcalfe, 2009).Paleozoic–Mesozoic granitic rocks outcrop extensively on HainanIsland, accounting for �40% of the island’s land area (Fig. 2)(Wang et al., 1991). Approximately 60% of these rocks are thoughtto be of Indosinian (Triassic) age, and the remainder of Yanshanian(Jurassic and Cretaceous) age (Li et al., 2006; Wang et al., 1991).The Indosinian granitoids are mainly unfoliated, medium- tocoarse-grained monzogranite with abundant K-feldspar mega-crysts that outcrop primarily on the Central Island. Strongly foli-ated granitoids outcrop over an area of ca. 800 km2; most ofthese are exposed in the central and southern parts of HainanIsland. In these rocks, abundant mafic magmatic and paragneissicenclaves occur in sizes from several tens of centimeters up to tenmeters. Large areas of Cenozoic basalts are distributed in the northpart of Hainan Island (Fig. 2). The incipient volcanism in northernHainan likely occurred in the Late Oligocene and increased gradu-ally in intensity toward the Miocene and Pliocene (Ho et al., 2000).Paleozoic–Mesozoic sedimentary rocks are widely developed inwest Hainan and are scattered in the east Hainan. Paleozoic sedi-mentary rocks are shallow-marine strata and characterized bysandstone, slate, and limestone. In contrast, Mesozoic sedimentaryrocks are terrestrial strata and characterized by volcanic-clasticrocks. Additionally, Quaternary sediments are widely distributedin the coastal plain around Hainan Island.

The landscape of Hainan Island is characterized by a centralmountainous region (attaining a maximum elevation of 1867 m)surrounded by low hills, a broad basaltic mesa (in the northernpart) and coastal marine-built terraces. As a result, Hainan Islandis characterized by a radial river system, mostly originating fromthe central mountainous area with high elevation (Fig. 1b). The

Sanya

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Fig. 2. Geological map (modified by Shi et al., 2011) of Hainan Island with spat

Nandu (length of 311 km), Changhua (230 km), and Wanquan(163 km) Rivers are the three largest rivers on Hainan Island: theircatchment areas together account for 47% of the island area. Intotal, rivers on Hainan Island deliver approximately 31 km3 ofwater and 4 Mt of sediment into the SCS (Zhang et al., 2013), mostof which occurs during Typhoon season (July–October) (Yang et al.,2013).

The climate of Hainan Island is dominated by tropical mon-soons, with a dry season from November to April (winter) and awet season from May to October (summer). The vegetationconsists of typical tropical broadleaf forests and grass-forb com-munities in the high elevation areas (>500–1000 m), and scrub-dominated and cultural crop landscapes in the lowlands. Theannual average temperature of Hainan Island is within the rangeof 22.8–25.8 �C and the annual rainfall ranges between 960 and2140 mm/a (Fig. 2), with �80% of that occurring between Mayand October. There is a large rainfall difference between the westand east sides of Hainan, resulting from the orographic effect ofthe south central highlands. Annual precipitation tends to decreasewestward from more than 2000 mm in central and east Hainan toapproximately 1200 mm in southwest Hainan (Fig. 2). In general,the eastern part of Hainan is dominated by a warm-wet-windy cli-mate, whereas there is a relatively dry-windy climate in northwestHainan and a dry-hot-windy climate in southwest Hainan (He andZhang, 1985; Xu et al., 2013). Hainan Island is frequently andseriously affected by tropical cyclones (TC) (Liu and Chan, 2003).From 1959 to 2000, the island was impacted by an annual meanof 7.9 TCs and was directly hit by 2.6 TCs (Huang, 2003). TCsmainly impact Hainan Island between June and November. Theassociated precipitation is Hainan Island’s main water source,accounting for more than one-third of the total precipitation (Wuet al., 2007).

3. Sampling and analytical methods

A total of 54 riverine samples were collected downstream ofestuary sites from 20 major rivers on Hainan Island in November2013 (Fig. 1b). These samples were obtained from surface muddychannels or bed deposits to avoid contamination from bank sedi-ments. All samples were analyzed for clay minerals at the Key Lab-oratory of Marine Sedimentology and Environmental Geology,State Oceanic Administration of China, using a D/max-2500 diffrac-tometer with Cu Ka radiation (40 kV and 100 mA). Three XRD runswere performed on oriented mounts of non-calcareous clay sized

aikou

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Paleozoic sedimentary rocksMesozoic sedimentary rocksCenozoicsedimentary rocks

Paleozoic-Mesozoic granites

CenozoicvolcanicsRainfall (mm/a)

ial distribution of annual averaged rainfall (modified by Long et al., 2011).

Table 1Clay mineral assemblages of surface sediments from the Hainan Island.

Regions Rivers Sample Smectite (%) Illite (%) Kaolinite (%) Chlorite (%) Illite chemical index Illite crystallinity (�D2h)

East Hainan Nandu R. NDJ-1 0 25 43 32 0.63 0.37NDJ-3 0 17 59 23 0.78 0.36NDJ-4 2 24 46 28 0.81 0.30NDJ-5 1 24 52 22 0.67 0.37NDJ-6 2 15 56 27 0.71 0.29NDJ-7 0 13 50 38 0.82 0.31

Wenchang R. WCH-1 0 10 61 29 1.02 0.39WJH-1 0 4 61 36 1.06 0.29

Wenjiao R. WJH-2 0 6 54 40 0.92 0.31WQH-6 0 12 58 30 0.93 0.31

Wanquan R. WQH-7 0 16 54 29 0.65 0.36WQH-8 0 19 51 29 0.80 0.33

Jiuqu R. JQJ-1 0 33 36 31 0.84 0.81JQJ-2 0 25 40 35 1.17 0.59

Longgun R. LGH-1 0 31 35 35 1.07 0.40LGH-2 0 26 44 30 0.43 0.36

Longtou R. LTH-2 0 6 59 35 1.03 0.33Lingshui R. LSH-1 0 8 57 35 0.56 0.43

LSH-2 0 8 58 34 0.57 0.44Shentian R. STH-1 2 4 66 28 0.70 0.37

STH-2 6 6 57 32 0.74 0.38Tengqiao R. TQH-3 1 10 52 37 0.67 0.35

SW Hainan Ningyuan R. NYH-1 5 18 48 29 0.51 0.39NYH-2 3 24 50 23 0.49 0.37NYH-3 4 21 50 26 0.49 0.34

Baisha R. BSH-1 11 13 50 26 0.38 0.32BSH-2 15 10 49 26 0.47 0.31

Nangang R. NGH-1 10 16 49 24 0.55 0.36NGH-2 12 17 38 33 0.57 0.33

Gan’en R. GEH-1 4 31 31 34 0.52 0.27GEH-2 8 28 35 29 0.56 0.33CHJ-1 7 20 51 22 0.41 0.34CHJ-2 6 23 43 28 0.36 0.41CHJ-3 5 20 41 34 0.42 0.37

Changhua R. CHJ-4 8 22 46 25 0.44 0.39CHJ-5 8 20 43 29 0.50 0.37CHJ-6 9 22 41 28 0.45 0.38

NW Hainan Zhubi R. ZBJ-1 2 20 51 26 0.74 0.42ZBJ-2 4 21 50 25 0.72 0.39ZBJ-3 4 18 56 22 0.63 0.40ZBJ-4 4 18 56 22 0.63 0.40

Paipu R. PPQ-1 8 16 46 30 0.63 0.40PPQ-2 8 18 45 29 0.64 0.39

Beimen R. BMJ-1 1 12 57 30 0.75 0.42BMJ-2 1 14 50 36 0.56 0.38BMJ-3 2 15 49 35 0.46 0.56

Chun R. CJ-1 7 16 42 34 0.68 0.52CJ-2 15 17 36 32 0.61 0.42

Wenlan R. WLJ-1 6 19 50 26 0.56 0.39WLJ-2 8 15 49 27 0.61 0.36WLJ-3 4 5 61 30 1.18 0.33WLJ-4 15 10 50 25 0.72 0.32WLJ-5 4 7 59 30 1.06 0.36

Table 2Average clay mineral assemblages of various sub-provinces in the Hainan and surrounding drainage basins of the Northern South China Sea.

Locations Smectite (%) Illite (%) Kaolinite (%) Chlorite (%) Illite chemical index Illite crystallinity (�D2h) Reference

E Hainan 1 16 52 32 0.80 0.38 This studySW Hainan 8 20 44 28 0.48 0.35NW Hainan 6 15 50 29 0.70 0.40Taiwan 5 53 5 37 0.33 0.16 Liu et al. (2010b)Pearl 5 31 46 18 0.62 0.30Luzon 87 1 5 7 – –Mekong 11 35 28 26 0.47 0.21Red 7 44 26 24 0.40 0.20

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(<2 lm) particles, following air-drying, ethylene–glycol solvationfor 24 h, and heating at 490 �C for 2 h. Identification of clay miner-als was made mainly according to the position of the (001) series

of basal reflections on the three XRD spectra. Semi-quantitativeestimation of clay mineral abundances is based on the peak areasof smectite (17 Å), illite (10 Å), and kaolinite/chlorite (7 Å) on the

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Fig. 3. (a) Ternary diagram of the major clay mineral groups illite + chlorite,kaolinite, and smectite and (b) Correlation of kaolinite/(illite + chlorite) with illitechemical index, showing the different forcing processes on the clay mineralassemblages in the surrounding areas of the SCS. Data of the Pearl, Red and MekongRivers from Liu et al. (2007b), the SW Taiwan rivers from Liu et al. (2008), the Luzonrivers from Liu et al. (2009b), and the Malay Peninsula, Sumatra, and Borneo riversfrom Liu et al. (2012b).

88 B. Hu et al. / Journal of Asian Earth Sciences 96 (2014) 84–92

glycolated curve using the MDI Jade 6.5 software. Relative propor-tions of kaolinite and chlorite were determined based on the ratioof the 3.57/3.54 Å peak areas. In order to compare with literaturedata from other rivers surrounding the SCS and the SCS shelf/slope(Liu et al., 2007b, 2008, 2009a,b, 2010b, 2012b), the Biscayeweighting factors are not used when calculating relative weightpercentages of each clay mineral. The error on the reproducibilityof semi-quantitative evaluation is estimated to be ±5% for each claymineral.

In addition, the illite chemistry index (the ratio of illite 5 Å and10 Å peak areas) and illite crystallinity [the full width at half max-imum height (FWHM) of the illite 10 Å peak] were determined onthe glycolated curve. Illite chemistry indexes below 0.15 representFe–Mg-rich illites (biotite, mica) characterized by physical erosion,whereas indexes above 0.4 are primarily found in Al-rich illites(muscovite) formed by strong hydrolysis (Petschick et al., 1996).Lower (higher) values of illite crystallinity represent good (poor)crystallinity and indicate weak (strong) hydrolysis and arid andcold (humid and warm) climate conditions in continental sources(Chamley, 1989; Ehrmann, 1998). These parameters have beenwidely used to track source regions and to estimate the intensityof chemical weathering (Petschick et al., 1996).

4. Results

According to the climatic and geological conditions of HainanIsland, we grouped the 20 studied rivers into three different prov-inces: east Hainan (10 rivers), southwest Hainan (5 rivers), andnorthwest Hainan (5 rivers) (Table 1).

The clay mineral assemblages of 22 samples from ten rivers ineast Hainan are dominated by kaolinite (35–66%, average 52%),with moderate chlorite (22–40%, average 32%), less abundant illite(4–33%, average 16%), and very scarce smectite (0–6%, average<1%). The illite chemical index in east Hainan varies between0.43 and 1.17, with most higher than 0.6, with a mean of 0.80. Illitecrystallinity ranges from 0.29 to 0.81�D2h, with an average value of0.38�D2h.

The southwest Hainan riverine sediments (15 samples from fiverivers) consist of relatively less kaolinite (31–51%, average 44%),comparable illite (10–31%, average 20%), and higher chlorite (22–34%, average 28%) and smectite (3–15%, average 8%) comparedwith those of east Hainan. The illite chemical index in southwestHainan is usually low, approximately 0.36–0.57, with a mean of0.48. Illite crystallinity in this province varies slightly from 0.27to 0.47�D2h, with an average value of 0.35�D2h.

In northwest Hainan, the clay mineral components of 16 sam-ples from five rivers are generally similar to those of southwestHainan, with slightly more kaolinite (36–61%, average 50%) andless illite (5–21%, average 15%). However, the values of the illitechemical index (0.46–1.18, mean of 0.70) and illite crystallinity(0.32–0.56�D2h, mean of 0.40�D2h) are increased relative to thoseof southwest Hainan.

In summary, the clay mineral assemblages of the Hainan river-ine sediments display three distinct characteristics (Table 2): (1)high kaolinite (35–66%), illite chemical index (0.43–1.18), and illitecrystallinity (0.29–0.81�D2h) in east and northwest Hainan; (2)low kaolinite (31–51%), illite chemical index (0.36–0.57), and illitecrystallinity (0.27–0.41�D2h) in southwest Hainan, and (3) minorsmectite (mean of 7%) in west Hainan (including southwest andnorthwest Hainan), with smectite being nearly absent in eastHainan.

5. Discussion

5.1. Clay mineralogical changes in the Hainan Island Rivers

Generally, the differences in clay mineral compositions arerelated to the weathering intensity, which is mainly determinedby climatic conditions (rainfall, temperature) and geological set-tings (lithology and morphology) (Chamley, 1989; Garzanti et al.,2014; Li et al., 2012a; Liu et al., 2007b, 2012b; Wang et al., 2011;Yang, 1988). Clay minerals in soils originate mainly from theweathering products of parent rocks (Wilson, 1999). Both thephysical and chemical weathering processes play important rolesin the formation of clay minerals: physical weathering leads torock fragmentation, whereas chemical weathering involves thesubtraction of ions, which in turn produces new minerals. Duringthe chemical weathering processes, the more mobile ions (e.g.,Na, K, Ca, Mg, and Sr) are expelled first from minerals in the parentrocks (bisialitization), the transitional elements (Mn, Ni, Cu, Co,and Fe) tend to be evacuated later (monosialitization), and Si isremoved last (alitization) (Chamley, 1989).

Kaolinite is usually found in monosialitic soils, representing theintensive hydrolysis under warm and humid climate conditions(Galán and Ferrell, 2013). Kaolinite contents of the Hainan riverinesediments vary from 33% to 66%, with a mean of 49%. Under thecombined effects of a warm and humid monsoon climate, stabletectonics, and good vegetation conditions, parent rocks enriched

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in alkali and alkaline elements (e.g., granite, granodiorite) on Hai-nan are easily and intensively weathered to form kaolinite. Similarsituations also occur in the Pearl River (Liu et al., 2007a, 2007b)and in tropical Southeast Asian (Malay Peninsula, Sumatra, andSouth Borneo rivers) (Liu et al., 2012b; Wang et al., 2011).

Illite and chlorite are primary minerals and are interpreted toform by weak hydrolysis and/or strong physical erosion of bedrockunder relatively dry climatic conditions (Galán and Ferrell, 2013).Illite and chlorite together contribute 46% of the Hainan riverinesediments, which is comparable with the Pearl River (49%), butmuch lower than the Mekong (61%), Red (68%), and Taiwanese(90%) Rivers (Table 2). Illite in the Hainan riverine sediments couldbe derived from the direct physical erosion of metamorphic andgranitic parent rocks that result from the intense seasonal precip-itation. Moreover, the relative high contribution of chlorite is likelyrelated to the bedrock types of Hainan, where metamorphic,metasedimentary and magmatic rocks outcrop widely and can pro-duce abundant chlorite by precipitation-caused physical erosion inthe highlands. Although they are the abundant primary mineralson Hainan Island, illite and chlorite actually suffer moderate to

Pearloffshore

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0 10 20 30 40 50 60Kaolinite (%)

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PearlK=46%IC=0.22

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Eastern NE SCS

(b)

?

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Fig. 4. (a) Ternary diagram of the clay mineral groups illite + chlorite, kaolinite, andsmectite of the northern SCS surface sediments and potential source areas endmembers. (b) Correlation of illite crystallinity and kaolinite of surface sediments inthe northern SCS and surrounding drainage basins. Dash lines indicate linearcorrelations between Taiwan and Pearl/Hainan end members. In addition to theHainan, other clay mineral data are from Liu et al. (2010b). IC = illite crystallinity(�D2h), K = kaolinite (%).

strong chemical weathering as confirmed by the high illite chemis-try index and illite crystallinity (Table 2).

Smectite is a secondary mineral in soil and is formed in theearly weathering stage of unstable Fe–Mg–Ca-rich minerals inigneous or metamorphic rocks (Galán and Ferrell, 2013). Formationof smectite in soil requires warm and wet climatic conditions withpoor drainage, which leads to soil solutions that have high pH andare concentrated in silica and basic cations (Righi and Meunier,1995; Velde, 1995). Although the sedimentary lithology settingsbetween east and west Hainan are almost the same, moderate tominor smectite (1–15%) mostly occurs in west Hainan, where theannual evaporation significantly exceeds precipitation (Zhanget al., 2006). Strong heavy rainfall caused by TCs may cause moreefficient formation of kaolinite in the east Hainan (Table 2).

The Hainan-sourced clay mineral assemblage is similar to thatof the Pearl River in South China: both of them consist dominantlyof kaolinite (�50%) and illite + chlorite (�46%) and are scarce insmectite (�4%) (Table 2 and Fig. 3a). In contrast, the Hainan river-ine clay mineral compositions are obviously different from otherbasins (Fig. 3a). Different forcing processes (climatic, tectonic,and lithological) play competitive roles in the formation andpreservation of clay minerals in these basins, which will be furtherdiscussed in the next section.

5.2. Weathering intensity on Hainan Island compared with othersurrounding areas in the northern SCS

Clay minerals in riverine sediments can provide valuable infor-mation about weathering types and intensity at the basin scale(Chamley, 1989; Fagel, 2007; Singer, 1980; Thiry, 2000). Recentstudies focused on other basins surrounding the SCS have shownthat multiple factors (climatic, tectonic, and lithological) play sig-nificant roles in the weathering processes (Fig. 3) (Liu et al.,2007b, 2009b, 2012b; Wang et al., 2011). In principle, kaolinite isgenerally formed by chemical weathering, while illite and chloriteare mostly inherited from parent rocks by physical erosion. Thus,the kaolinite/(illite + chlorite) ratio indicates the type of weather-ing (chemical weathering vs. physical erosion) that affected theerodible sediments, with increased values suggesting strong chem-ical weathering and weak physical erosion, and vice versa (e.g.,Alizai et al., 2012; Boulay et al., 2007; Sionneau et al., 2010;Colin et al., 2010; Huang et al., 2011; Thamban et al., 2002; Wanet al., 2010a). In addition, two crystallographic indices of illite(illite chemical index and illite crystallinity) are also widely usedto assess the chemical weathering intensity and to constrain sedi-ment provenances (Chamley, 1989; Ehrmann et al., 2007; Liu et al.,2007b, 2008, 2012b; Petschick et al., 1996; Wan et al., 2010a,2010b, 2012; Wang and Yang, 2013). A series of recent studieshave confirmed that these indices are correlated to the chemicalweathering state of the riverine sediments surrounding the SCS(Liu et al., 2007b, 2008, 2009b, 2012b).

Values of the illite chemistry index range between 0.36 and1.18 for all river samples on Hainan Island (Table 1), with highervalues occurring in east Hainan (mean of 0.80), moderate valuesin northwest Hainan (mean of 0.70), and lower values in south-west Hainan (mean of 0.48) (Table 2). Except for one sample fromthe Changhua River (CHJ-2, 0.36), all values above 0.40 indicatethat Al-rich illites found on Hainan are predominately formedby strong hydrolysis. Thus, Hainan Island, from a clay mineralogystandpoint, is characterized by moderate to intensive weatheringprocesses (Fig. 3). This is also confirmed by the poor illite crystal-linity (mean of 0.38�D2h) and high kaolinite contents (mean of49%) of the Hainan riverine sediments (Table 2). Compared withother basins surrounding the SCS (Fig. 3b), the weathering statusof Hainan is comparable with those of the Pearl River in SouthChina (Liu et al., 2007b) and the Malay Peninsula, Sumatra and

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trait

Beibu Bay

Fig. 5. Spatial distribution of the illite + chlorite contents (%) (white line) and the associated ocean currents in the northern SCS (modified by Liu et al., 2010b). Detailed datapoints of surface sediment can be found in Liu et al. (2010b). GDCC = Guangdong Coastal Current, SCSWC = South China Sea Warm Current, SCSBK = South China Sea Branch ofKuroshio, KC = Kuroshio Current, DWC = Deep Water Current.

90 B. Hu et al. / Journal of Asian Earth Sciences 96 (2014) 84–92

South Borneo basins in tropical Southeast Asia (Liu et al., 2012b).All of these basins are benefited by long-term warm and humidclimatic conditions and stable tectonic settings, which produceshigh kaolinite contents (generally >50%) and illite chemicalindexes (ca. 0.5–0.7), indicating dominant chemical weatheringwith strong hydrolysis. In contrast, Taiwan and North Borneo,and to a lesser extent, the Red and Mekong Rivers are character-ized by physical erosion due to the combined effects of tectonicuplift and abundant monsoon precipitation (Li et al., 2012a; Liuet al., 2007b, 2008, 2012b). Illite and chlorite are the dominantclay minerals in these basins (generally >60%), and they are extre-mely high in Taiwan and N Borneo (up to �90%), with an illitechemical index lower than 0.4 (Fig. 3).

Furthermore, obvious differences exist in the three sub-prov-inces of Hainan Island, with higher values of kaolinite, illite chem-ical index, and illite crystallinity in east and northwest Hainan thanthose of southwest Hainan (Table 2), which indicates that moreintense chemical weathering occurs in east and northwest Hainan.This regional discrepancy is most likely due to the contrast inannual precipitation (Fig. 1b) that results from frequent typhoon-derived heavy rainfall in east and northwest Hainan (Guo et al.,1993), as well as the orographic effect of the south-central high-lands that blocks water vapor from the SCS on the east side. There-fore, our clay mineralogy results suggest that rainfall could be theprimary factor controlling the chemical weathering intensity onHainan Island. Similar meteorology situations can also be foundon Taiwan Island, however, they produce opposite results. Accord-ing to Li et al. (2012a), the values of the illite chemical index in thewest Taiwan Rivers are higher than those in east Taiwan, whichimplies that chemical weathering intensity is greater in the westTaiwan. The east Taiwan Rivers are more mountainous, with stee-per erosional gradients, shorter transport distances, and shorterresidence times. Therefore, typhoon-derived heavy rainfall triggersmore intensive physical erosion, while chemical weathering isdiminished (Li et al., 2012a). This scenario suggests the tectonicuplift and associated steeper relief have substituted climatic forc-ing (i.e., rainfall) as being the controlling factor of the weatheringprocesses on Taiwan Island.

5.3. Implications for sediment provenance discrimination in theNorthern SCS

In recent years, many works have focused on the clay mineralassemblages of surface sediments (e.g., Li et al., 2012b; Liu et al.,2010a, 2011, 2012a, 2013; Liu et al., 2008, 2010b) and core sedi-ments (e.g., Boulay et al., 2005, 2007; Huang et al., 2011; Hu et al.,2012; Liu et al., 2003, 2004, 2005, 2010c; Steinke et al., 2008; Wanet al., 2008, 2010a, 2010b) in the SCS, aiming to constraint the claymineral sources and transport pathways, as well as to elucidateinformation about the related paleoenvironmental and paleoclimat-ic conditions. As for the northern SCS, where Hainan Island is situ-ated, three discriminative clay mineral end members, i.e., Taiwan,Pearl River, and Luzon, have been recognized, and the detrital fine-grained sediment contributions from each of them have beensemi-quantitatively evaluated (Liu et al., 2008, 2010b). The basicinterpretation of Liu et al. (2008, 2010b) is that most smectite inthe Northern SCS derives from the Luzon arc system and most kao-linite sources from the Pearl River, whereas illite and chlorite origi-nate from both Taiwan (with low illite crystallinity) and the PearlRiver (with high illite crystallinity) (Fig. 4 and Table 2). Accordingly,Liu et al. (2008) estimate that the contributions of clay minerals are52% from the Pearl River, 29% from Taiwan, and 19% from Luzon tothe South China shelf, and 31% from the Pearl River, 23% fromTaiwan, and 46% from Luzon to the South China slope. However,subsequent studies (Liu et al., 2010a, 2011, 2013) suggest that thePearl River sediments are mostly limited to the area betweenthe Pearl River mouth and the southeast of Hainan Island due tothe blocking effect of ocean currents.

Recent works (Ge et al., 2014; Liu et al., 2009a, 2014) indicatedthat the Pearl River has formed a 400 km elongated, shore-parallelHolocene mud deposit, extending from the Pearl River delta south-westward off the Guangdong coast to the Leizhou Peninsula(Fig. 5). The formation mechanism is similar to that of the Yellowand Yangtze River-derived sediments in the East China Sea (i.e.,summer deposition and winter transport) (Yang et al., 1992).During the summer, the Pearl River sediments entering the estuaryare mostly trapped within the shelf area near the estuary. In the

B. Hu et al. / Journal of Asian Earth Sciences 96 (2014) 84–92 91

winter, these initially deposited sediments can be re-suspendedand reworked by winter storms, and then be transported to thesouthwest by the strengthened Guangdong Coastal Current (GDCC)(Fig. 5) (Ge et al., 2014; Liu et al., 2014). Thus, the modern PearlRiver fine-grained sediments were mostly trapped in the delta/estuary area and/or delivered southwestward to form the muddeposit (Ge et al., 2014; Liu et al., 2009a, 2014). This impliesthat when sea level reached a certain depth during the glacial-interglacial cycles, the contribution of the Pearl River fine-grainedsediments to the northern SCS shelf and slope should be largelyreduced, if not terminated. This is further evidenced by the claymineral changes of core KNG05, located on the northern SCS slope(Fig. 5), which indicate that the Pearl River was the major clay min-eral source during the late glacial period (17.5–14.5 ka), when sealevel dropped by approximately 100 m, whereas Taiwan has beenthe major contributor since �12.5 ka (Huang et al., 2011).

Assuming the Pearl River fine-grained sediments have beentrapped within the coastal area (<�50 m water depth), as sug-gested by Ge et al. (2014), then other substituted clay mineralend members with high kaolinite contents are necessary to recon-cile the spatial distribution of kaolinite in the surface sediments ofthe northern SCS (Liu et al., 2010b), because both Taiwan andLuzon provide only scarce kaolinite (Table 2) (Liu et al., 2008,2009b). Considering the clay mineral results in this study and theregional oceanic current pattern in the northern SCS, HainanIsland, which has been ignored in the previous studies, may bethe most probable ‘missing’ end member. As shown in Fig. 5, thespatial distribution of illite + chlorite contents in the northernSCS displays a double tongue-shaped pattern extending fromTaiwan offshore to the southwest along the �100 m and 2000–2500 m isobaths, which corresponds well to the flow routes of win-ter Guangdong Coastal Current (GDCC) and to the subsurface SouthChina Sea Branch of Kuroshio (SCSBK) and Deep Water Current(DWC) through the Luzon Strait, respectively (Fig. 5) (Liu et al.,2010b). However, there is also a small but distinct tongue ofillite + chlorite extending from southeast Hainan northeastwardsalong the continental slope (100–200 m isobath). This most likelyindicates the dispersal pathway of the Hainan fine-grained sedi-ments, transported by the northeastward South China Sea WarmCurrent (SCSWC, Fig. 5). High kaolinite content has been docu-mented on the southeast of Hainan Island and is preliminarilythought to be delivered from the Pearl River (Liu et al., 2010a,2011). However, a local anti-cyclonic eddy exists throughout theyear on the east Leizhou Peninsula (Guan and Yuan, 2006; Yanget al., 2002), which effectively traps the southward Pearl River-derived fine-grained sediments and forms a distal depocenter (Geet al., 2014). The high kaolinite contents near the southeast partof Hainan Island are most likely sourced from the small rivers onHainan Island. Moreover, the spatial variability of clay mineralsin the northern SCS, especially in its western territory, can alsobe well explained by changing the respective contributions fromHainan, Taiwan, and Luzon (Fig. 4). Our results thus suggest thatHainan Island may be an important clay mineral source area, espe-cially during the high sea level stage (e.g., Holocene). Further inves-tigations of the geochemical characteristics (e.g., REE, Sr-Nd-Pbisotopes, trace elements) and other provenance proxies of HainanIsland riverine sediments are needed to confirm this proposal.

6. Conclusion

Our results indicate that the clay mineral assemblages of Hai-nan consist dominantly of kaolinite (31–66%), with a lesserabundance of chlorite (22%–44%) and illite (4%–33%), andscarce smectite (0–15%). The east and northwest Hainan riverinesediments are characterized by higher kaolinite contents, illite

chemical index, and illite crystallinity than those of southwest Hai-nan. Minor smectite (mean of 7%) occurs in west Hainan and smec-tite is nearly absent in east Hainan. As a whole, Hainan Island ischaracterized by moderate to intensive chemical weathering withstrong hydrolysis. Rainfall is the principal forcing factor controllingthe chemical weathering intensity on Hainan Island, with moreintense chemical weathering occurring in east and northwest Hai-nan. By combining the clay mineral compositions of surroundingbasins and surface sediments in the northern SCS, we suggest thatthe Hainan fine-grained sediment carried by the South China SeaWarm Current in the summer likely plays an important role in pro-viding clay minerals to the northern SCS.

Acknowledgments

We thank Zuofen Chen, Xiaohui Han and Yuankai Xiong fromthe Marine Geology Survey of Hainan for assistance in collectingthe samples. We especially thank Editor J.G. Liou and anonymousreviewers for their constructive reviews on the early version of thispaper. This study was jointly supported by the National Key BasicResearch Program of China (2013CB429701), Ministry of Land andResources (1212011088112), the National Natural Science Founda-tion of China (41206049, 41476052, 41306066, and 41376079).

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